1. Field of the Invention
The present invention relates to an optical fiber, and more specifically, relates to an amplifying optical fiber and a method for fabricating the same.
2. Description of the Related Art
While in a trivalent ionic state rare-earth elements, such as, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, and Yb may emit fluorescence by electron transition. An optical fiber containing the rare-earth element may have a function of amplifying an input optical signal by means of a stimulated emission effect. By appropriately regulating reflectance at both ends of the optical fiber, it may also have a function of an optical fiber laser continuously producing the stimulated emission. Typically, within a light communication band, Pr3+, Nd3+ and Dy3+ emit fluorescence in the wavelength range of 1.3 to 1.4 μm, Tm3+ emits fluorescence in the wavelength range of 1.4 to 1.5 μm, Er3+ emits the fluorescence in the wavelength range of 1.5 to 1.6 μm. Therefore, an Er3+ doped optical fiber amplifier is widely used for the wavelength range of 1.5 to 1.6 μm within a light communication band.
In the wavelength range of 1.5 to 1.6 μm, an Er3+ doped optical fiber amplifier produced by adding Er3+ to commonly-used quartz glass optical fiber, is used. However, an optical fiber amplifier for emitting fluorescence in wavelength ranges of 1.3 to 1.4 μm or 1.4 to 1.5 has not been put to practical use because the fluorescence emitting efficiency of Pr3+, Dy3+ and Tm3+ in quartz glass is too low in those wavelength ranges. As an alternative to avoid those problems, U.S. Pat. No. 5,071,460 entitled “Process for the Preparation of fluoride glass and process for the preparation of optical fiber perform using the fluoride glass”, issued to Kazuo Fuziura, and U.S. Pat. No. 5,567,219 entitled “Polyimide coated heavy metal fluoride glass fiber and method of manufacture”, issued to Lubos Vacha, disclose a method for fabricating the amplifying optical fiber by adding the rare-earth element to a fluoride glass optical fiber so as to promote the fluorescence emitting efficiency.
However, using the fluoride glass incurs problems in that unlike existing silica glass it can not produce basic glass material with high purity by a chemical vapor deposition method and in that it is difficult to control the refractive index difference between a core and a cladding within the range of 0.1 percent. As an alternative to the problem, a process for fabricating fluoride amplifying optical fiber by using an over-jacketing method is disclosed.
FIGS. 1 and 2 shows the method for fabricating the fluoride amplifying optical fiber by using the over-jacketing method known in the prior art. Referring to FIGS. 1 and 2, one portion corresponding to a core of the amplifying optical fiber is made in the form of a rod 110 and the other portion corresponding to a cladding thereof is made in the form of a tube 120. The rod 110 is inserted into a hole 125 formed in the tube 120 to produce the basic material in the form of a rod. Afterward, by using an elongating apparatus for the fluoride amplifying optical fiber, the fluoride optical fiber is elongated.
On the other hand, U.S. Pat. No. 6,128,430 entitled “Composition for optical waveguide article and method for making continuous clad filament” and U.S. Pat. No. 6,374,641 entitled “Method of making an optical fiber by melting particulate glass in a glass cladding tube”, both of which were issued to Polly Wanda Chu, disclose a method comprising a step of fluorinating a quartz glass component in quartz tube, in other words, substituting the quartz to be used as core component with fluoride, so as to improve the fabricating process.
FIG. 3 illustrates a view of a fluoride amplifying optical fiber known in the prior art. The fluoride amplifying optical fiber 200 comprises a core 210 disposed at a center of the fiber and a cladding 220 surrounding the core 210. As can be appreciated from a refractive index distribution curve 230, because there is a significant refractive index difference between the core 210 and the cladding 220, a problem occurs in that optical signals transmit in a multi-mode 240, 250. If a core diameter A of the fluoride amplifying optical fiber 200 is reduced below 4 μm in order to transmit the optical signal in a single mode, coupling loss increases significantly when the fluoride amplifying optical fiber is coupled to the quartz amplifying optical fiber, the diameter of which is typically 8 μm.
As stated above, the amplifying optical fiber known in the prior art includes problems outlined below.
First, when transmitting through the amplifying optical fiber having a refractive index difference between the core and the cladding, light waves transmit in a multi mode.
Second, if the core diameter of the fluoride amplifying optical fiber is reduced below 4 μm, when coupled to the quartz optical fiber for transmitting light, coupling loss increases greatly.
Third, because the O—H chemical bond in the fluoride glass increases when the fluoride glass is exposed to moisture, light-wave loss increases and the mechanical strength of the optical power decreases, and thus reliability of the amplifying optical fiber deteriorates.
Fourth, when quartz glass is substituted with fluoride glass, a problem occurs in that light scattering loss resulting from oxi-fluoride core composition increases significantly.